Project Summary/Abstract Antimicrobial resistance is a significant public health concern, and persistent use of antimicrobial drugs in food animal production systems has been implicated as an important factor contributing to its development. The expanding threat of antimicrobial resistance, which has been recognized both domestically (CDC, 2013) and internationally (WHO, 2015), has prompted regulatory actions aimed at preserving efficacy of antimicrobial drugs. In January of 2017, the U.S. Food and Drug Administration completed its implementation of Guidance for Industry# 213, fully enacting policy that disallows use of medically-important antimicrobials as growth promotants. This policy restricts use of certain medically important antibiotics to therapeutic applications under the supervision of a licensed veterinarian. Tylosin phosphate is among the medically important classes of compounds affected by this policy. Tylosin has been used extensively by the beef feedlot industry for decades to reduce incidence of liver abscesses, and generally is regarded as the most efficacious method for controlling liver abscesses in cattle. Presence of abscesses at harvest is a primary cause for liver condemnation, and thus has significant economic ramifications. Severe abscesses can impact liver function, thereby decreasing animal performance and jeopardizing animal well-being. Additionally, abscesses can expand to affect adjacent tissues, requiring affected areas to be excised from the carcass at a significant economic loss, and also decreasing processing speed at a cost to abattoirs. Oversight of tylosin usage in feedlots has shifted to veterinarians in response to changes in regulatory policy, but actual usage patterns in commercial feedlots have remained relatively unchanged. In the absence of alternative measures for control of liver abscesses, drastic changes in usage patterns seem unlikely to occur. Tylosin currently is approved for continuous feeding only, though there are indications that more targeted durations of use may have efficacy equal to that of continuous feeding. Whereas replacement may not be a viable option at present, reduction of use through strategic application presents itself as a meaningful alternative. Commercial implementation of alternative feeding strategies for tylosin will require labeling changes that provide for greater flexibility in its application. With this in mind, our proposed project has several aims, the first of which is to provide additional insight relative to the timing of liver abscess formation in feedlot cattle through non-invasive, ultrasound imaging. Secondly, we will evaluate two different feeding strategies aimed at decreasing tylosin use, while maintaining health outcomes similar to those achieved with continuous exposure.